Why do Worms Come out in the Rain?

Some people may be surprised to learn the real reason worms come out in the rain, if for no other reason than what they thought or learned as children is often wrong. Earthworms appear by the dozens after it rains. We see them crawling around in the damp earth of a garden, on our lawns and even on the concrete of sidewalks, driveways and parking lots.

But why?

It’s certainly not because they want to sacrifice themselves to the birds! It’s nothing like that. In fact, there are several factors that go into the process, all of which to combine to make the soil and air conditions just right for an earthworm migration to the surface.

As children we may have heard someone explain the presence of so many worms like this: They will drown if they stay underground when the soil is so wet. This is not true and may actually be the opposite of reality.

Worms don’t breathe with lungs as human beings and other animals do. They take oxygen into their bodies through their outer skin. Some studies have shown that worms can actually live in water for an extended amount of time (hours, even a day or two) if the water has enough oxygen in it.

We can’t abandon the earthworm breathing process completely when we try to explain their presence on the surface after rain. It seems the earthworm’s outer layer has a mucus on it, a moist, somewhat slimy substance that helps them take in oxygen and expel the gases they don’t need. They have to remain moist for this process to work properly.

Except after rain, the worms wouldn’t thrive above ground because it’s too warm and too dry. It is really much better for them above ground after it rains and they simply take the opportunity to go to this new location while still being able to survive and thrive. But this doesn’t completely explain why earthworms travel above ground, because they could survive in the wet ground, as long as it isn’t truly flooded.

The answer seems to be that earthworms would rather mate on the surface. Scientists believe that coming out of the earth makes it easier for them to find a mate. This means they don’t have to work their way through many feet of earth while trying to locate this mate. The moist conditions after a rain is something like a signal to the worms that others will be “out and about.”

Close observation of earthworm activity has shown that worms even gather in groups (just a few at a time) when they travel up and out after a rain. So, earthworms use the ideal conditions to gather and mate. But they also take in the abundant oxygen in the air. Water-soaked soil may have oxygen but it may not be enough at some point.

Watch for earthworms after it rains. But there’s one more thing – you will probably see them at night, when there is no sun to dry them out.